The reformer contains about 200-400 tubes made off chromium nickel steel, 10-13 meters long, with an inner diameter of 75-140mm and a wall thickness of 11-18mm (Ullmann’s, 2011 p.175). Under the severe conditions taking place in the reformer the tubes start to creep and rupture. Determining factors for such deformation are the internal pressure and the tube-wall temperature. Figure shows the stress to rupture of different materials. To avoid tube rupture, lower pressures are employed, leading to higher power use for compression in ammonia synthesis. The reaction in the reformer is endothermic and proceeds with an increase in volume. To compensate for the lower conversion rate due to the increased pressure the reaction temperature needs to increase. However, tube materials limit the allowable increase in temperature (Ullmann’s, 2011 p.175). 

The standard material for many years was the HK 40. The HP modified (1.5% Nb) material, due to its improved temperature properties (40 bar reforming pressure at a tube wall temperature of 900^oC), has been used in many tube replacements and tubes in new plants (Ullmann’s, 2011 p.175). The use of microalloys containing Ti and Zr are another improvement (see Figure 11). Their use permits the reduction of tube-wall thickness while maintaining the same tube lifetime (100,000 hours). By installing tubes with smaller wall thickness and/or wider diameter the capacity of the front end of an ammonia plant can be increased (Beyer et al., 2005 p.7).

The use of microalloyed tubes with minimum wall thickness can increase the catalyst volume, increase firing and lower the pressures drop (A2A Toolkit). According to Fertilizer Association of India, with the new improvements in materials it is possible to increase the life time of the reformer tubes (by up to 20 000 hours) and have higher tube-wall temperatures (up to 906 ^oC) (FAI, 2013).